Constants

constants

When you do not want others (or yourself) to override existing variable values, use the const keyword. This will declare the variable as "constant", which means unchangeable and read-only.

Mutable variable

int a = 2;
a = 3;
std::cout << "a : " << a << '\n';

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Constants as a promise

const int b = 3;
// b = 4; <- this fails. `b` is constant.
std::cout << "b : " << b << '\n';

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const from mutable value

// `b2` can be initialized with any value, but cannot be changed after this.
// This is possible, but makes it impossible to know b2 in compile-time.
const int b2 = a;
std::cout << "b2 : " << b2 << '\n';

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Constant "more constant than const"

// `c` can only use values available at compile-time
// `c` cannot be initialized with a mutable value
constexpr int c = 4;
std::cout << "c : " << c << '\n';

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Constant "more constant than const"

// `c` can only use values available at compile-time
// `c` cannot be initialized with a mutable value
constexpr int c = 4;
std::cout << "c : " << c << '\n';

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Creating arrays from mutable value

// The compiler needs to know the size at compile time
// Impossible:
// std::array<int,a> v1;

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Creating arrays from compile-time const values

// Sometimes possible, if b was not initialized with a mutable value
std::array<int, b> v2 = {5, 6, 5};
std::cout << "v2.size(): " << v2.size() << '\n';

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Creating arrays from runtime const values

// Impossible (b2 depends on a variable):
// std::array<int,b2> v2b;

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Creating arrays from compile-time constexpr values

// Always possible
std::array<int, c> v3 = {7, 8, 7, 8};
std::cout << "v3.size(): " << v3.size() << '\n';

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